What Could Have Been: Original plans for a sequel to Frankenstein were much different. Two other possible plots were conceived, one being the Monster continuing Dr. Frankenstein's research and the other one being Dr. Frankenstein inventing a death ray in the eve of World War I.
The role of Dr. Pretorius was offered to Claude Rains, but he was attached to another project at the time and couldn't participate. They also offered it to Bela Lugosi, but he turned it down.
The film was initially planned to be called The Return of Frankenstein.